The invention relates to a rotor for an asynchronous machine, and to an asynchronous machine having a rotor of this kind.
Electrical machines, such as asynchronous machines, are used as electrical drive assemblies of motor vehicles or utility vehicles, such as, for example, electric vehicles, hybrid vehicles, etc.
In said applications, there is a tendency to operate the asynchronous machines at relatively high rotation speeds with relatively low torques. In order to achieve a greater power density in this case, it is desirable to reduce the dimensions of the asynchronous machine. However, this is possible only to a limited extent for the following reasons.
Known rotors of asynchronous machines are generally constructed from a laminated core, which is arranged on a corresponding shaft of the rotor, and a cage which passes through the laminated core. The cage contains a large number of rods which pass through the laminated core and are electrically connected to one another by a short-circuiting ring at the respective end sides of the laminated core.
The laminated core is subjected to mechanical loading during operation by the centrifugal forces which occur. The centrifugal force which occurs can be described by the following equation:F=m*ω2*r 
As is clear, the centrifugal force F decreases as the overall size or the radius r of the rotor falls, but rises quadratically as the angular velocity ω increases.
In addition to the centrifugal force-dependent loading of the laminated core, temperature-related loading phenomena also occur. The cage of the rotor is generally manufactured from copper and therefore has a greater coefficient of thermal expansion than the laminated core. Consequently, the short-circuiting ring and rods of the cage expand to a greater extent than the laminated core when there is an increase in temperature during operation of the asynchronous machine.
Both the centrifugal force-dependent loading phenomena and also the temperature-related loading phenomena increase in the direction of the end sides of the laminated core and there lead to increased mechanical stresses.
The mechanical stresses which occur necessarily cause corresponding dimensioning of the webs, that is to say the region of the laminations which is located on the outside of the rods in the radial direction, and therefore limit the desirable reduction in the dimensions of the rotor and, respectively, of the asynchronous machine.
Considerations of increasing the size of the web by the cage in the laminated core being designed to be smaller radially in relation to the rotation axis fail in that less power is achieved in this way.
Expressed in general, the problem is that the maximum possible rotation speed or the maximum possible reduction in the radial dimensions of the rotor at this rotation speed is limited owing to the strength of the webs in the region of the end sides.
Approaches which increase the rotation speed of an asynchronous machine are known from document DE 102012214772 A1. This document describes a rotor for an asynchronous machine, which rotor supports a laminated core which accommodates a complete cage which is formed in a die-casting process and has rods and corresponding short-circuiting rings which connect the rods. One or more steps are formed in the transition region between the respective rotor rods and the corresponding short-circuiting ring, said steps leading to a reduction in the notch effects in the transition region and, respectively, the local stress peaks and therefore allow higher rotation speeds.
However, the strength of the web of the corresponding laminated core is not addressed in said document.
Rather, owing to the formation of the steps in the transition region between the rotor rods and the corresponding short-circuiting ring, the mass of the cage is further increased in this region, and for this reason the problems of the temperature-dependent and centrifugal force-dependent loading phenomena already explained above are not reduced in a rotor of the kind described in said document.
In this respect, there is also the problem with regard to said document that the maximum possible rotation speed or the maximum possible reduction in the radial dimensions of the rotor at said rotation speed is limited by the required strength of the webs in the region of the end sides.
Furthermore, the production costs are increased in the case of the known rotor since the individual laminations of the laminated core have to be punched differently in order to realize the steps.
Against the above background, the object of the present invention is to provide a rotor for an asynchronous machine, which rotor permits a further increase in the rotational speeds and, respectively, a reduction in the dimensions in comparison to the prior art. One objective of the invention is, at least, to provide an alternative rotor to the prior art.
The object is achieved by a rotor in accordance with embodiments of the invention.
A rotor which is constructed according to one aspect of the invention and is intended for an asynchronous machine contains a laminated rotor core and a rotor cage.
The laminated rotor core of the rotor is constructed from a large number of rotor laminations which are layered in a longitudinal direction of the rotor, an intended rotation axis of the rotor running in said longitudinal direction.
The laminated rotor core is preferably mounted or fitted on a shaft of the rotor, wherein the rotation axis runs through the shaft. When the rotor is used as intended, said rotor is inserted into a stator of an asynchronous machine and is mounted in the stator such that the rotor can rotate about the rotation axis which runs through the shaft.
The layered rotor laminations are preferably fitted to or stacked on one another. The fitting or stacking can preferably be performed by a force-fitting and/or interlocking connection, for example by so-called punch-stacking. Furthermore, the layered rotor laminations can be fitted to or stacked on one another by a cohesive connection, such as an adhesive connection for example.
The rotor cage of the stator has a large number of rotor bars, which run through the rotor laminations in the longitudinal direction, and at least one short-circuiting ring (also called shorting rings) which is arranged at an end of the laminated rotor core, which is situated in the longitudinal direction, in such a way that said short-circuiting ring electrically connects the rotor bars to one another.
The rotor laminations are preferably punched before the stacking in each case to form openings, wherein the rotor laminations are oriented during the stacking such that the resulting laminated rotor core has a large number of passages. The rotor bars are inserted into these passages and run through the laminated rotor core as a result. A short-circuiting ring which electrically connects the rotor bars to one another is arranged at least at one end of the laminated rotor core. Here, the short-circuiting ring preferably bears directly against an end side of the laminated rotor core.
The rotor bars and the short-circuiting ring can be secured to one another for example by a soldered or welded connection. As an alternative, the rotor bars and the short-circuiting ring can be drawn into the laminated rotor core in one piece in a die-casting process.
A short-circuiting ring is preferably arranged not only at the one end of the laminated rotor core, but also at the other end of the laminated rotor core, wherein, in this case, the further short-circuiting ring at the other end likewise preferably bears against a corresponding end side of the laminated rotor core.
The rotor bars and the short-circuiting ring(s) are preferably formed from copper, a copper alloy, aluminum, an aluminum alloy or a special alloy.
The laminated rotor core contains at least one rotor lamination in a region at the end, said rotor lamination having a greater strength and/or a greater rigidity in one radial direction with respect to the rotation axis than the other rotor laminations.
Owing to this inventive design of the laminated rotor core and, respectively, the arrangement of the at least one rotor lamination having the greater strength and/or greater rigidity in the region at the end of the laminated rotor core at which the short-circuiting ring is arranged, greater centrifugal forces and, respectively, the greater stresses which occur in the region, can be better absorbed at the webs.
As a result, the maximum possible rotation speed of the asynchronous machine which is equipped with the rotor can be further increased or the radial dimensions of the rotor can be further reduced given the same maximum rotation speed in comparison to the case of all of the rotor laminations having a uniform thickness in the longitudinal direction and being formed from an identical material.
In general, the performance of the asynchronous machine can be increased by the rotor according to the invention given a more compact construction. The more compact construction is accompanied by a savings in weight.
The higher strength is achieved in that the at least one rotor lamination is formed from a material with a higher strength than the other rotor laminations. During operation as intended, the laminated rotor core and the corresponding rotor laminations are subject to mechanical tensile stress mainly by the centrifugal forces which occur. In this respect, the strength is understood to mean, in particular, the tensile strength, and the rigidity is understood to mean the tensile rigidity.
In addition to the strength or as an alternative, the rigidity of the at least one rotor lamination is greater than that of the other rotor laminations.
The greater rigidity can be achieved, for example, by a greater thickness of the at least one rotor lamination in the longitudinal direction or a special geometric configuration of the at least one rotor lamination.
In this respect, the at least one rotor lamination can, for example, be formed from the same material as the other rotor laminations, wherein the corresponding rigidity of said at least one rotor lamination is increased in comparison to that of the other rotor laminations by way of its thickness or geometric configuration.
As mentioned, the at least one rotor lamination can, as an alternative or in addition, be formed from a material with a greater strength in comparison to that of the other rotor laminations.
The region at the end of the laminated rotor core deforms to a lesser extent during operation of the rotor on account of the increased strength and/or rigidity, and for this reason the air gap between the rotor and the stator can also be reduced for the purpose of saving on installation space.
The other rotor laminations are, in particular, those which are arranged in a center of the laminated rotor core in the longitudinal direction.
The rotor is preferably configured such that the region at the end has a large number of rotor laminations which have the greater strength and/or the greater rigidity.
Here, the rotor laminations of the large number of rotor laminations can be of identical or different configurations. In particular, the strengths and/or the rigidities of the large number of rotor laminations can be designed such that they are matched to a stress profile, which occurs during operation of the rotor, in the laminated rotor core.
The region at the end of the laminated rotor core preferably has, starting from the end, a length in the longitudinal direction of from 10% to 20% of a total length of the laminated rotor core.
The at least one rotor lamination or the rotor laminations of the large number of rotor laminations is/are formed from a material with a yield strength of from 550 MPa to 650 MPa.
By way of example, high-strength electrical metal sheets have yield strengths of this kind.
Materials which have balanced electrical and mechanical properties can be used in general. “Structural steels” or tool steels can also be used if the losses in the respective application vary in an acceptable range.
The at least one rotor lamination preferably has a greater rigidity owing to the at least one rotor lamination having a greater thickness in the longitudinal direction than the other rotor laminations. Here, the at least one rotor lamination preferably has a thickness of from 0.4 mm to 1 mm, particularly preferably of from 0.50 mm to 0.65 mm, in the longitudinal direction.
Furthermore, the end of the laminated rotor core is preferably formed by a spring steel sheet.
The rotor is very particularly preferably configured such that the region at the end has a large number of rotor laminations which have the greater strength and/or the greater rigidity, each of the large number of rotor laminations is formed from a material with a yield strength of greater than/equal to 550 MPa, particularly preferably of from 550 MPa to 650 MPa, and the end of the laminated rotor core is formed by a spring metal sheet, in particular a spring steel sheet.
Furthermore, the rotor laminations of the large number of rotor laminations are preferably thicker, in particular 0.4 mm to 1 mm, particularly preferably from 0.5 mm to 0.65 mm thicker, than the other rotor laminations in the longitudinal direction.
The other rotor laminations are, in particular, standardized rotor laminations which are manufactured from a material with a yield strength of approximately 380 MPa and have a thickness in the longitudinal direction of approximately 0.2 to 0.35 mm.
The rotor is further preferably configured in such a way that the rotor cage has a further short-circuiting ring which is arranged at another end of the laminated rotor core in the longitudinal direction in such a way that it electrically connects the rotor bars to one another; wherein the laminated rotor core contains at least one rotor lamination or a large number of rotor laminations in a region at the other end, said rotor lamination having a greater strength and/or a greater rigidity in a radial direction with respect to the rotation axis than the other rotor laminations which are not located either in the region at the one end nor in the region at the other end.
The above statements in respect of the configuration of the region at one end of the laminated rotor core equally apply for the configuration of the region at the other end of the laminated rotor core.
Furthermore, the invention relates to an asynchronous machine, in particular an asynchronous motor, which functions as a drive assembly, for a motor vehicle, comprising a rotor as has been described in the text above.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.